Zirconium-Catalyzed Amine Borane Dehydrocoupling and Transfer Hydrogenation

Abstract

A B S T R A C T : κ 5 -( M e 3 S i N C H 2 C H 2 ) 2 N -(CH 2 CH 2 NSiMe 2 CH 2 )Zr (1) has been found to dehydrocouple amine borane substrates, RR′NHBH 3 (R = R′ = Me; R = t Bu, R′ = H; R = R′ = H), at low to moderate catalyst loadings (0.5−5 mol %) and good to excellent conversions, forming mainly borazine and borazane products. Other z i r c o n i u m c a t a l y s t s , ( N 3 N ) Z r X [ ( N 3 N ) = N -(CH 2 CH 2 NSiMe 2 CH 2 ) 3 , X = NMe 2 (2), Cl (3), and O t Bu (4)], were found to exhibit comparable activities to that of 1. Compound 1 reacts with Me 2 NHBH 3 to give (N 3 N)Zr-(NMe 2 BH 3 ) (5), which was structurally characterized and features an η 2 B−H σ-bond amido borane ligand. Because 5 is unstable with respect to borane loss to form 2, rather than β-hydrogen elimination, and 2−4 do not exhibit X ligand loss during catalysis, dehydrogenation is hypothesized to proceed via an outer-sphere-type mechanism. This proposal is supported by the catalytic hydrogenation of alkenes by 2 using amine boranes as the sacrificial source of hydrogen. ■ INTRODUCTIONApplication of amine boranes in materials science, for hydrogen storage, and in organic synthesis demonstrates the usefulness of these simple Lewis acid−base adducts as materials precursors and chemical reagents. 1−5 The most pointed driver for recent study of amine borane dehydrogenation has been the potential use of these molecules for hydrogen storage, owing to their high hydrogen content by weight and relative ease of hydrogen loss. However, thermal degradation of amine boranes is poorly controlled and time-consuming. 3,6 Thus, catalysts that operate under desirable conditions (i.e., mild temperatures and faster reaction times) to form products in a controlled manner have been sought. Some of the most active catalysts that have been reported are capable of operating at ambient temperatures using rare and expensive group 9 transition metals such as iridium and rhodium. 7−10 This is a highly active field of study, and catalysts from across the periodic table have been reported including transition-metal and main group compounds. 11−19 Group 4 catalysts were some of the earliest studied for amine borane dehydrocoupling and have elicited interest from several groups due to their high reactivity and mechanistic richness. Manners, 20−22 Chirik, 23 Rosenthal, 24 Wass, 25,26 Beweries, 24,27 and Baker 28 have all presented detailed studies of related group 4 metallocene complexes. These independent studies indicate that titanium compounds demonstrate greater activity than their heavier congeners and that increased electron donation and steric pressure from ancillary ligands decrease reactivity. Substrate selectivity was also observed for group 4 metallocenes, which demonstrated high activity for secondary amine borane dehydrocoupling, while these compounds were almost inert toward ammonia borane. The lower activity of these metallocene compounds with NH 3 BH 3 is perhaps due to competing formation of stable amido borane complexes that precipitate out of soluti...

Modern applications of low-valent early transition metals in synthesis and catalysis

“…Remarkably, this reaction is reversible, opening up the possibility that these complexes might be useful as hydrogenation catalysts. In fact, 45 also performs a unique example of early-transition-metal-promoted transfer hydrogenation 196 (FIG. 5a, bottom left), hydrogenating cyclohexene with concomitant production of 46 , with the ligand serving as the terminal hydrogen donor.…”

“…Despite computational evidence for a M II /M IV redox mechanism for amine borane dehydrogenation 212 , there is conflicting experimental evidence that indicates alternate mechanisms may also be involved. For example, several M IV precatalysts such as M(NMe 2 ) 4 (M = Ti, Zr) 208 , [ i Pr 2 Si(NDIPP) 2 ] Zr(NMe 2 ) 2 -LiCl(THF) 3 (DIPP = 2,6-diisopropylphenyl) 213 and K 5 -(Me 3 SiNCH 2 CH 2 ) 2 N(CH 2 CH 2 SiMe 2 CH 2 ) Zr (REF 196 ) may promote dehydrogenation via c-bond metathesis routes, while in other cases, M III metallocene species have been observed in situ 214 .…”

Ionic Liquid Facilitated Dehydrogenation of tert-Butylamine Borane

“…However, it takes less time when compared to several metal catalyst-facilitated dehydrogenations. 5,6 However, for diammoniate diborane (DADB), which is formed during the dehydrogenation of AB, Stowe et al 25 identifies the −BH 2 resonance at δ = −13 ppm. Similarly, Neiner et al 26 observed the −BH 2 resonance at δ = −10 ppm that originates from the bimolecular EDAB resulting from the thermal dehydrogenation of pure EDAB.…”

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

“…The inverse hydrogenation process of the obtained products has also been explored [419][420][421][422][423]. Several metal catalysts have been investigated for the dehydrocoupling of these hydrogen reservoirs [424][425][426][427][428][429][430][431]; among these, pincer complexes have showed promising features and remarkable activity [432][433][434][435][436][437][438][439][440]. A comprehensive review on the main progresses in this field can be found in the work of Rossin and Peruzzini [441], whereas Schneider investigated the ruthenium-catalyzed amino-borane dehydrocoupling in 2013 [442].…”